Thermodynamics of the two-dimensional easy-axis quantum antiferromagnet

The thermodynamics of the two-dimensional quantum Heisenberg antiferromagne t with easy-axis anisotropy (EA-QHAF) on the square lattice is studied by m eans of the semiclassical approach of the pure-quantum self-consistent harm onic approximation (PQSCHA). The anisotropic model shows a phase transition at finite temperature with critical exponents of the two-dimensional Ising model for every value of the anisotropy. Our theoretical results are compa red with the experimental data on the real compound Rb2MnF4 both above (cor relation length) and below (staggered magnetization) the transition: a good agreement is found in both the disordered and the ordered phase. (C) 2001 Elsevier Science B.V. All rights reserved.